Bright solder plating

ABSTRACT

Disclosed is a method and composition for electroplating a layer of bright solder on a base metal. In addition to soluble divalent tin and lead salts, the bath contains formaldehyde, an aryl primary amine, a condensation product of an aromatic aldehyde and an aromatic amine and a polynuclear aromatic disulfonic acid. Deposits obtained by this bath are not only mirror bright, but exhibit no haze. An important advantage is that the solderability of the deposit may be correlated to its physical appearance.

United States Patent [1 1 Karustis, Jr. et al.

BRIGHT SOLDER PLATING [75] Inventors: George A. Karustis, Jr.,Westfield;

' Elizabeth P. Leahy, North Caldwell,

both of NJ.

[73] Assignee: Oxy Metal Finishing Corporation,

Warren, Mich. a [22] Filed: May 21, 1973 [21] Appl. No.: 362,595

[52] US. Cl. 204/43 S [51] Int. Cl C23b 5/38 [58] Field of Search.....,204/43 S, 54 R, 53

, [56] I References Cited 1 I UNITED STATES PATENTS 3,66l',730 5/1972Nishihara 204/43 S Nov. 26, 1974 3,730,853 5/1973 Sedlacek et all.204/43 S 3,749,649 7/1973 Valayil 204/43 S Primary Examiner-G. L. KaplanAttorney, Agent, or FirmArthur E. Kluegel [57] ABSTRACT Disclosed is amethod and composition for electroplating a layer of bright solder on abase metal. In addition tosoluble divalent tin and lead sallts, the bathcontains formaldehyde, an aryl primary amine, a condensation product ofan aromatic aldehyde and an aromatic amine and a polynuclear aromaticdisulfonic acid. Deposits obtained by this bath are not only mirrorbright, but exhibit no haze. An important advantage is that thesolderability of the deposit may be correlated to its physicalappearance.

7 Claims, N0 Drawings BRIGHT SOLDER PLATING BACKGROUND OF THE INVENTIONThis invention relates to the electrodeposition of solder on a basemetal. More specifically, it relates to the electrodeposition of mirrorbright solder.

The two most common methods of obtaininga layer of solder on a substrateare melt-plating and electroplating. The melt-plating technique isundesirable 'for many applications because of the high temperaturesrequired and because the thickness of the deposited layer is uneven anddifficult to control. As a result of these shortcomings, electroplatingtechniques have become more and more important particularly with theadvent of the printed circuit industry. Conventional techniques forelectroplating solder provide an electrolyte containing divalent tin anddivalent lead, commonly as the fluoborate salts, in addition to excessfluoboric acid and formaldehyde. Summaries of both the electroplatingand melt-plating processes are set forth in U.S. Pat. No. 3,661,730, thecontent of which is incorporated herein by reference.

SUMMARY OF THE INVENTION The present invention provides an aqueousacidic composition suitable for plating a bright tin-lead alloy aromaticdisulfonic acid or salt, and a surfactant comof high solderability. Inaddition, it provides a method of plating such an alloy on a base metalsurface.

One of the major problems in solder plating is that slight variations inthe bath composition, particularly variations in the ratio of the tin tolead in the bath, will result in unsatisfactory products. Specifically,if the tin and lead are not within the proper ratios, the resultingalloy will not have the desired crystal structure and, as a result, whenmelted, the deposit will not effectively solder the base metal to asecond metal. In the past, the only reliable means for determiningwhether a given deposit was, in fact, solderable was to melt an actualplated sample and test the sample for adhesion. The present invention,in addition to providing a mirrorbright solder plate, also provides ameans for visually determining whether the deposit is acceptable withoutactually melting it. When the bright, low-haze deposits ofour inventionare obtained, good solderability can be reliably predicted. This is avaluable feature from the quality control standpoint.

The plating composition of this invention contains between 3.5 and 60grams per liter of divalent tin, preferably between 12 and 30 grams perliter. It also contains divalent lead in a range of between 30 and 4 5weight percent, based on the total of the tin and lead in the bath.Since lead is insoluble in the presence of most anions, care must betaken not to include any anions in the bath which would precipitate thelead. While other soluble compounds may be used, the preferred compoundto be added to the bath are the fluoborate salts of tin and lead. Lesspreferred are the methane bination.

DETAILED DESCRIPTION OF THE INVENTION Formaldehyde should be present inthe composition in an amount between 6 and 12 grams perliter, preferablyin an amount between 8 and 10 grams per liter. It. is conveniently addedas formalin, a 37 percent aqueous solution of formaldehyde. The arylprimary amine should be present in an amount of at least 0.1 grams perliter, preferably between 0.1 and 0.9 grams per liter, and mostpreferably between 0.3 and 0.6 grams per liter. Amines suitable for thispurpose may be represented by the following formula:

R NHz wherein R is selected from a group consisting of alkyl andhydrogen; Ar is an aryl radical having up to 2 fused rings substitutedor not with alkyl or alkoxy groups having up to 4 carbon atoms orhalide; and wherein the two substituents of the above formulaare orthoto each other. The condensation product therefore may. be eview dPzUleesae formula Ar Rs It has been found that th condensaTion productof 2-hydroxy naphthalene-l-carboxaldehyde and oethylaniline is thepreferred compound of this type since it minimizes the amount offormaldahyde required in the electrolytic solution. This bath compo nentshould be present in an amount of from 0.001 grams per liter up to thesolubility limits of the solution. Normally, the concentration will bebetween 0.01 and 0.1 grams per liter. The product of the above mentionedcondensation reaction has the following structural formula:

Methods for the preparation of such condensation products may be foundin Chemistry of the Carbon- Nitrogen Double Bond, Sol Patai, Editor,Inter Science (1970), or in similar books on organic-nitrogen chemistry.

The polynuclear aromatic disulfonic acid component must have at leasttwo fused aromatic rings and two sulfonic acid or sulfonatesubstituents. in the case of the naphthalene derivative, naphthalene-2,7disulfonic acid disodium salt is preferred. While only a very smallamount of this component is necessary in order to obtain a brightdeposit, if it is omitted inferior results are obtained. Amounts rangingfrom less than 0.001 grams per liter up to 0.02 grams per liter aresufficient while amounts between 0.001 and 0.005 grams per liter arepreferred.

The surfactant combination necessary to obtain a bright solder depositmust include an anionic surfactant, a nonionic surfactant, as well as anamphoteric surfactant. Suitable anionic and amphoteric surface activeagents may be selected from any of those found in handbooks on thesubject. A preferred anionic surfactant is Aerosol 22, trademark for aproduct manufactured by the American Cyanimid Company, and comprising a35 percent active solution of tetrasodium N- (l,2-diocarboxyethyl)-N-octadecyl-sulfosuccinamate. Preferred amphoteric surfactants includeMiranol HM a substituted imidazoline having the approximate formula:

CHzOOON a manufactured by the Miranol Chemical Company, and Triton QS-l5manufactured by Rohm 8L Haas Company which contains the sodium salt ofasulfonated tertiary amine. Of the non-ionic surfactants, the only onefound effective is Luprintol P manufactured by BASF- Wyandotte which isa polyethoxylated benzyl phenol. The surfactants may be present in traceamounts or more.

Excess quantities of surfactants are not desirable because they provideno further improvement in the resulting solder plate. Optimum resultscan normally be obtained with concentrations of each of these types ofsurfactants ofless than 1 gram per liter. Suitable plating conditionsare those conventional in the art, for example, as disclosed in theaforementioned US. patent. Typically, the temperature of the platingbath will be about room temperature or below and the current densitywill be at least 12 A.S.F., preferably less than 50 A.S.F., and mostpreferably about 30 ASP. Additional additives may be included in thebath for varying pur-- EXAMPLES In each experiment, the organics wereadded to a standard bath containing about 2 oz/gal (15 g/l) tin as thedivalent cation, 1% oz/gal (10 g/l) lead as the divalent cation, 23oz/gal (172.5 g/l) fluoboric acid and 2 oz/gal (15 g/l) boric acid. ThepH of such a solution is below 1. The tin and lead were added as thefluoborates to avoid introduction of interfering anions.

Test panels were steel having a polished copper plate of approximately50 millionths of an inch thick. Test circuit boards were copper-platedand covered with a resist except for 8 parallel lines having thru-holesat each end thereof.

HNCEA is a shorthand notation for the condensation product of 2-hydroxynaphthalene-l-carboxaldehyde with o-etheylaniline and NDAS is shorthandfor naphthalene-2,7 disulfonic acid disodium salt.

The following were added to the standard bath:

Test panels and circuit boards were plated at 30 A.S.F. for 10 minutesat room temperature using one bagged anode, a rectifier with 5 percentripple or less and with rocker arm agitation. The solder deposit wasmirror bright and exhibited a slight haze. Similar tests without NDASyielded solder deposits having a much more noticeable haze. When melted,the solder exhibits excellent adherence.

Additional experiments were performed varying the concentration of theingredients. Deposit quality was found to be noticeably superior whenthe concentrations were within the ranges specified in the foregoingdescription.

What is claimed is:

1. An aqeuous acidic composition suitable for electroplating a brighttin-lead alloy of high solderability, comprising:

a. 3.5 60 g/l divalent tin;

b. 30 45 wt. percent divalent lead, based on the total of lead and tinin the bath;

c. 6-l2 g/l formaldehyde;

d. at least 0.1 g/l of an aryl primary amine;

e. at least 0.001 g/l of a condensation product of an aromatic aldehydeand an aromatic amine; and

f. a surfactant combination comprising:

1. an anionic surfactant; 2. an amphoteric surfactant; and 3. anon-ionic surfactant.

2. The composition of claim 1 additionally comprising at least 0.001 g/lof a polynuclear aromatic disulfonic acid or salt thereof.

3. The composition of claim 2 wherein said sulfonic acid comprises atleast two fused aromatic rings and two sulfonic acid substituents, saidrings substituted or not with alkyl or alkoxy groups of up to fourcarbon atoms or a halide.

4. The composition of claim 1 wherein said aryl primary amine has theformula;

wherein R through R are independently selected from the group consistingof alkyl and alkoxy groupsof up to four carbon atoms, hydroxy, halideand hydrogen; R

is selected from the group consisting of alkyl and hydrogen, Ar is anaryl radical having up to two fused rings, substituted or not withhalide, alkyl or alkoxy groups having one to four carbon atoms.

6. A method of electroplating solder on a ,base metal surface comprisingelectrolyzing the composition of claim 1 with said base metal ascathode.

7. The method of claim 6 wherein said composition additionally comprisesat least 0001 g/l of a polynuclear aromatic disulfonic acid.

1. AN AQUEOUS ACIDIC COMPOSITION SUITABLE FOR ELECTROPLATING A BRIGHTTIN-LEAD ALLOY OF HIGH SOLDERABILITY, COMPRISING: A. 3.5-60 G/L DIVALENTTIN; B. 30-45 WT. PERCENT DIVALENT LEAD, BASED ON THE TOTAL OF LEAD ANDTIN IN THE BATH; C. 6-12 G/L FORMALDEHYDE; D. AT LEAST 0.1 G/L OF ANARYL PRIMARY AMINE; E. AT LEAST 0.001 G/L OF A CONDENSATION PRODUCT OFAN AROMATIC ALDLEHYDE AND AN AROMATIC AMINE; AND F. A SURFACTANTCOMBINATION COMPRISING:
 1. AN ANIONIC SURFACTANT;
 2. A AMPHOTERICSURFACTANT; AND
 2. The composition of claim 1 additionally comprising atleast 0.001 g/l of a polynuclear aromatic disulfonic acid or saltthereof.
 2. an amphoteric surfactant; and
 3. a non-ionic surfactant. 3.The composition of claim 2 wherein said sulfonic acid comprises at leasttwo fused aromatic rings and two sulfonic acid substituents, said ringssubstituted or not with alkyl or alkoxy groups of up to four carbonatoms or a halide.
 3. A NON-IONIC SURFACTANT.
 4. The composition ofclaim 1 wherein said aryl primary amine has the formula:
 5. Thecomposition of claim 4 wherein said condensation product is representedby the formula:
 6. A method of electroplating solder on a base metalsurface comprising electrolyzing the composition of claim 1 with saidbase metal as cathode.
 7. The method of claim 6 wherein said compositionadditionally comprises at least 0.001 g/l of a polynuclear aromaticdisulfonic acid.